Fixed "make docs" for out-of-tree builds.

Also: this automatically updates the version number in the docs, so I don't
 have to with each release.
(transplanted from 72778f1e98f06c1e02f88dc8d263027d61469766)

CMakeLists.txt

@@ -560,7 +560,23 @@ INSTALL(FILES src/physfs.h DESTINATION include)
 
 FIND_PACKAGE(Doxygen)
 IF(DOXYGEN_FOUND)
-    ADD_CUSTOM_TARGET(docs ${DOXYGEN_EXECUTABLE} COMMENT "Building documentation")
+    SET(PHYSFS_OUTPUT_DOXYFILE "${CMAKE_CURRENT_BINARY_DIR}/Doxyfile")
+    CONFIGURE_FILE(
+        "${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile"
+        "${PHYSFS_OUTPUT_DOXYFILE}"
+        COPYONLY
+    )
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "\n\n# Below auto-generated by cmake...\n\n")
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "PROJECT_NUMBER = ${PHYSFS_VERSION}\n")
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "OUTPUT_DIRECTORY = ${CMAKE_CURRENT_BINARY_DIR}/docs\n")
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "\n# End auto-generated section.\n\n")
+
+    ADD_CUSTOM_TARGET(
+        docs
+        ${DOXYGEN_EXECUTABLE} "${PHYSFS_OUTPUT_DOXYFILE}"
+        WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+        COMMENT "Building documentation in 'docs' directory..."
+    )
 ELSE(DOXYGEN_FOUND)
     MESSAGE(STATUS "Doxygen not found. You won't be able to build documentation.")
 ENDIF(DOXYGEN_FOUND)

docs/Doxyfile

@@ -23,7 +23,8 @@ PROJECT_NAME           = physfs
 # This could be handy for archiving the generated documentation or 
 # if some version control system is used.
 
-PROJECT_NUMBER         = 2.0.0
+# (CMake will set this properly at build time. --ryan.)
+PROJECT_NUMBER         = unknown_version
 
 # The OUTPUT_DIRECTORY tag is used to specify the (relative or absolute) 
 # base path where the generated documentation will be put.